The present invention relates to a bonding apparatus and bonding method, suitable to wire bonding and inner lead bonding used in assembling electronics parts or components.
In assembling stages of electronic parts or components, either wire bonding is performed to connect electrodes of a semiconductor chip with electrodes (or terminals) of a substrate by way of very narrow wire, or inner lead bonding is carried out to join very narrow leads formed on a carrier tape with electrodes (or terminals) of a semiconductor chip.
The conventional wire bonding will now be described as one of these prior art bonding techniques.
FIG. 6 is a schematic block diagram of a control system of the conventional wire bonding apparatus, and FIG. 7 is a timing chart for representing the conventional bonding method.
In FIG. 7, reference numeral 1 denotes a horn functioning as a bonding arm. A hollow capillary tool 2 is held by one end portion of this horn 1. A very narrow wire 3 is penetrated through this hollow capillary tool 2. The horn 1 is swung along upper and lower directions by means of a motor 1 (not shown).
In FIG. 6, reference numeral 4 denotes a calculator for calculating deviation "E" between an operation pattern signal "A" transferred from a control unit 5 and a feedback signal "B" transferred from a position detector (not shown) for detecting a rotation amount of the horn 1.
This operation pattern "A" is constructed of a plurality of pulse signals. The quantity of this pulse signal indicates the rotation amount of the horn 1, namely the transport distance of the capillary tool. The transport (travel) distance of the capillary is substantially proportional to the number of pulse signal. Also, the frequency of the pulse signal is proportional to the moving velocity (speed) of the capillary tube 2. The higher, the frequency of the pulse signal becomes, the faster the moving speed of the capillary tool 2 becomes. Conversely, when the frequency of the pulse signal is lowered, the moving speed of the capillary tool becomes slow. When the frequency is selected to be constant, the capillary tool 2 is transported at a constant velocity.
Similarly, a feedback signal "B" to the calculator 4 is arranged by pulse signals. Every time the capillary tool 2 is transported over a predetermined distance, the pulse signals are produced. As a consequence, the distance over which the capillary tool is traveled may be calculated by counting the quantity of pulse signals.
Reference numeral 6 indicates a drive circuit for driving a motor under such a control method selected from a "position control method", or a "torque control method". In case that the motor is driven under the position control method, a drive current I determined in accordance with the deviation "E" is supplied to the motor from this drive circuit. In other words, the motor is driven along such a direction that this deviation "E" becomes 0. On the other hand, the motor is driven by a drive current I defined in correspondence with a bonding load in response to a pressure applying instruction signal D in the torque control method. The changing operation of these control methods in the drive circuit 6 is carried out in response to a control method changing signal "C".
FIG. 8 represents a relationship between the deviation and the drive current in the drive circuit of the conventional bonding apparatus. The drive circuit 6 increases the value of the drive current "I" in accordance with the absolute value of the deviation "E".
A change in the drive currents "I" with respect to the deviation "E", namely an inclination degree of the curve shown in the relation diagram of FIG. 8, is varied in correspondence with the magnitude of the gain characteristic gain "K" of the drive circuit 6. In other words, when the gain characteristic "K" becomes high, an inclination of this curve becomes steep. Conversely, when the gain characteristic "K" becomes low, an inclination of this curve becomes gentle. Normally, the value of this gain characteristic "K" is set in such a range that the operation of the bonding apparatus becomes stable.
Very recently, there is a trend to set this gain characteristic "K" to a high value by either increasing the bonding speeds of the wire bonding apparatus, or by introducing the wire loop control function (function to control up/down movements of the horn so as to make the shape of the wire loop in a desired shape).
Referring now to FIG. 7, the conventional wire bonding method will be explained.
In FIG. 7, symbol "R" indicates a trail of operation positions of the lower end portion of the capillary tool 2. It should be understood that although FIG. 7 shows only the first bonding process for performing wire bonding to the electrodes or terminals of the semiconductor chip, since the second bonding process for executing wire bonding to electrodes or terminals of the substrate, this second bonding process is omitted.
First, after the ball 3a has been formed on the upper edge portion of the wire 3, the capillary tool 2 is moved at high speed "V1" downwardly toward the bonding surface S (surface of electrode of semiconductor chip, or surface of electrode of substrate). When the capillary tool 2 has reached a predetermined height SL (search level) from the bonding surface S, the transporting speed is reduced to the low speed "V2".
Subsequently, when a contacting detecting means detects such a fact that the ball 3a is in contact with the bonding surface "S" at the time instant T3, the transmission of the operation pattern signal A is interrupted from the control unit 5, and also the control method changing signal "C" is sent to the drive circuit 6 so as to change the control method from the position control method to the torque control. At the same time, the pressure applying instruction signal "D" is delivered to the drive circuit 6, so that the drive current "I" defined in correspondence with a predetermined bonding load is outputted from the drive circuit 6.
Next, during a time period from the time instant T3 to the time instant T4, the ball 3a is depressed by the capillary tool 2 against the bonding surface "S" at a preselected bonding load by way of torque produced from the motor. At the same time, the wire 3 is joined while ultrasonic waves produced from the ultrasonic vibrator mounted on the other edge portion of the horn 1 are applied to the bonding surface "S". Then, after the time instant T4 has elapsed, while the wire 3 is derived, the capillary tool 2 is elevated or lifted at the high speed to be transported toward the electrodes of the substrate, and the second bonding process is carried out, whereby 1 cycle process of the wire bonding operation is completed.
However, although the contacting detecting means detects such a fact that the ball 3a of the wire is in contact with the bonding surface S, actually, there is a time lag of approximately 5 milliseconds until the ball 3a is in contact with the bonding surface S at the time instant T2 and this contact condition is detected by the contact detecting means. As a result, during such a time period from the time instant T2 when the ball 3a is actually in contact with the bonding surface S until the time instant T3 when this contact condition by the ball 3a is detected, the control unit 5 continues to send the operation pattern signal "A" for instructing that the capillary tool is further continuously moved to descend.
Here, since the capillary tool 2 is contacted via the ball 3a onto the bonding plane S at the time instant T2, the movement of the capillary tool 2 is substantially stopped, and the pulse signals of the feedback signal B disappear at the time instant T2.
As a consequence, the deviation "E" is increased by the number of the operation pattern signal A1 which has been transmitted to the calculator 4 during the time interval from the time instant T2 to the time instant T3, as shown in FIG. 7, and the drive current supplied to the motor is also increased. In other words, the ball 3a is depressed against the bonding surface S under excessive force caused by torque produced by the increased drive current I. Accordingly, more great impact loads would be produced in conjunction with the impact loads occurring when the capillary tool 2 is landed and contacted onto the bonding surface S. Thus, according to the above-explained wire bonding method, such a measure has been taken that the speed V2 just before the capillary tool 2 is rounded onto the bonding plane S is further lowered in order not give such a damage that the ball 3a would be deformed and the bonding surface would be deformed. This may disturb such a high-speed bonding operation.